[FIX] mail: fix mock server mail channel write

Mock server's mail channel write method is incorrect. It assumes
the id of the channel will always be 20 while it could be anything.

closes odoo/odoo#86860

X-original-commit: 96e732f6bbc7622d50c3a969f5c2bb44c85c2b48
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/tests/helpers/mock_server.js

@@ -1215,7 +1215,7 @@ MockServer.include({
         this._widget.call('bus_service', 'trigger', 'notification', [{
             type: 'mail.channel/insert',
             payload: {
-                id: 20,
+                id,
                 avatarCacheKey: avatarCacheKey,
             },
         }]);